(1) Field of the Invention
The present invention relates to a glass composition, a protective-layer composition, and a binder composition as well as to a lamp employing the compositions stated above.
(2) Description of the Related Art
Among various types of lamps, fluorescent lamps are known to have high luminous efficiency. A fluorescent lamp is generally composed of a discharge tube or vessel which is filled with mercury and a rare gas, and of which inner surface is coated with phosphors. Upon discharge generated within the discharge tube, the mercury is excited to emit ultraviolet radiation having a wavelength mainly of 254 nm. Upon exposure to the thus generated ultraviolet radiation, the phosphors are excited to emit visible light. With this mechanism, the luminous flux of the fluorescent lamp is obtained. Conventionally known fluorescent lamps include a straight tube type, and a circular tube type in addition to a bulb-shaped type and a compact type, which are recently introduced to the market.
HID (High Intensity Discharge) lamps, on the other hand, collectively refer to high-pressure mercury discharge lamps, metal halide lamps, and high-pressure sodium lamps. A high-pressure mercury discharge lamp emits light upon discharge in mercury vapor under a pressure of 100 to 1000 kPa. A metal halide lamp emits light when discharge is generated to dissociate metal halide into metal atoms and halogen atoms, and the metal atoms are excited to emit visible light. A high-pressure sodium lamp emits light upon discharge in sodium vapor.
Fluorescent lamps and HID lamps as above are required to be low energy consuming, capable of generating high luminous flux, and long-lived. To achieve the required performance capabilities, various research and development are being made.
As one example, JP Publication No.11-167899-A discloses a technique for extending life of a fluorescent lamp. The technique is developed in view of the following drawback of a conventional fluorescent lamp employing soda glass. That is, during manufacturing or operation of a fluorescent lamp, sodium is eluted from the glass to react with mercury, which tends to reduce luminous intensity of the lamp. In order to suppress such reduction in luminous intensity, the disclosure teaches to employ glass having such a characteristic that alkali is less likely to be eluted in comparison with a conventional soda glass.
In another example, an attempt is made on a fluorescent lamp to obtain higher luminous flux by increasing luminous intensity of phosphors. Similar attempt is made by making a discharge tube thinner to increase the arc length.
Thanks to those research and development, fluorescent lamps and HID lamps have increased in performance to some extent. Yet, the recent demand for the performance capabilities is more intense than ever. To meet the demand, there is a need for techniques to further improve a lamp to be lower energy consuming, or to be capable of higher luminous flux.